The only thing I can suggest is trying to "wake" him for a longer feed. Oliver will doze off on the boob, but usually a gentle tap on the cheek, or a tickle on the back, etc will wake him enough so he keeps sucking. If that doesn't work I change his nappy which wakes him up, then I put him back on and he usually has another go. I find Oliver really only grizzles if he is hungry (even if it seems like I just feed him, if I put him back on he will feed like he is starving!) or has wind/gas. But yeah, my midwife said I should just feed him as much as he wants and try to wake him if he falls asleep too quickly.